Tanning is the process, which converts putrescible hide/skin into non-putrescible leather. Conventionally the raw or temporarily preserved hides/skins are first rehydrated well in a process called soaking and the soaked stock is subjected to liming to remove keratinous and other non-collagenous materials present in the raw hides and skins. At this stage, the hides/skins are known as pelt in the trade. The pelts are then subjected to various pre-tanning operations like deliming, bating, pickling, depending on the requirements to condition the same for the subsequent tannage. Further adjustment of pH is also required for fixing the tanning agent with collagen matrix, which is generally known as basification in the case of mineral tanning or fixing in the case of vegetable tanning.
Usually the tanning method primarily employs chromium or high molecular weight vegetable tannins for leather production. More than 90% of the leathers tanned globally contain chromium. The present commercial chrome tanning method gives rise to only about 50-70% chromium uptake as reported by Gauglhofer (Journal of the Society of Leather Technologists and Chemists, 70, 11, 1986). This poor uptake results in material wastage on one hand and ecological imbalances on the other. Buljan (World Leather, November, 65, 1996) reported that the international specification for the discharge of chromium bearing stream is less than 2 ppm. Even a high exhaust chrome tanning system does not provide such low discharge. The bio-toxicity of chromium(III) has been reported by Shrivastava and Nair (Biochemical and Biophysical Research Communications, 270, 749, 2000). Conversion of chromium(III) to chromium(VI) under oxidizing environment has been reported by Fathima et al (Journal of the American Leather Chemists Association, 96, 444, 2001). The disposal of chrome containing solid wastes and sludges is posing a major challenge as reported by Germann (Science and Technology for Leather into the Next Millennium, Tata McGraw-Hill Publishing Company Ltd., New Delhi, p. 283, 1999). Chromium(III) salt finds extensive usage in leather processing but the environmental concerns due to chromium pollution has led to the search for alternative tanning agents. Vegetable tanning, a natural material, has been considered as a suitable eco-friendly option to replace chromium and it is being employed for making some kinds of leathers. However, vegetable tanning leads to excessive loading in the leathers, which reduces its versatility to make different end products and also has low resource availability. Vegetable tannins are also known to be poorly biodegradable, which results in high biochemical oxygen demand (BOD) and chemical oxygen demand (COD).
Mineral tanning agents such as aluminium, titanium, iron, zirconium have been explored for their solo tanning potential for replacing chromium. However, each one has inherent disadvantages associated with them as highlighted by Madhan et al (Journal of the American Leather Chemists Association, 97, 189, 2002). Though aluminium and titanium tanning produces white leathers, the characteristics of the leathers are poor, especially hydrothermal stability. Iron tanned leathers undergo darkening of colour during ageing and also have poor strength characteristics. Zirconium tanning is known to produce leathers with drawn grain. Hence, the combination tannages were considered as suitable method to overcome the problems from single tanning system. Various combination-tanning systems exist in the world. Presently, the combination systems mainly based on less chrome and chrome free. Examples for less chrome combination tanning system are chromium and silica, chromium and iron, etc. Chrome free combination systems based on vegetable and metal ions other than chromium have been explored by Kallenberger and Hernandez (Journal of the American Leather Chemists Association, 78, 217, 1983). However, all these combination systems do not have commercial importance in the global leather industry due to processing difficulty, toxicity, availability, cost, etc.
The use of protein hydroylsates in leather processing as filling agent during post tanning has been reported by Chen et al (Journal of the American Leather Chemists Association, 96, 262, 2001) and as chrome exhaust aid by Ramamurthy et al (Journal of the Society of Leather Technologists and Chemists, 73, 168, 1989). Traubel et al (U.S. Pat. No. 6,254,644, 2001), manufactured biodegradable leather by tanning delimed pelt with aldehydes or bisulphite-blocked polyisocyanate. The use of an enzyme, transglutaminase, for tanning skins/hides, termed as enzymatic tanning, has been reported by Feigel et al (United States Patent Application 20020155524, 2002), whereby the hides are preferably treated with 0.5-10% transglutaminases and preferably aqueous solution at a pH between preferably 5 and 9 and at a temperature between 20 and 40° C. The major limitations associated with this process are high cost and its inability to produce variety of leathers as reported by Collighan et al (Journal of the American Leather Chemists Association, 99, 289, 2004). No prior art is available for tanning hides/skins with collagen based tanning agent or other related biological compounds.